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## Main Question or Discussion Point

In $9 after (9.1) in Landau's book <quantum mechanics>,I read a notation under there about (9.2) that: when operation i(Hf-fH) translate to classical limit, the first order is zero,and the second order is h[H,f],why? I just can't understand why the first order is zero(because u could substite sth to it and so Hf-fH=0?),and how does the second order be h[H,f],is there anything like pq=h here? I read Dirac's mechanics once but he did not tell it more clearly how the i and h appear, and here I was amazed again.thanks a lot!